System.UnauthorizedAccessException: Access to the path 'xxxx' is denied.

I am setting a machine with Windows Server 2008 R2. I have an asp.net in that tries to modify a xml file programatticlaly and I am getting error "System.UnauthorizedAccessException: Access to the path 'xxxx' is denied."
The strange thing about this is that I am able to add a new file and delete the file from that folder, only modification or overwrite on the current file is not working. I went through http://forums.asp.net/p/1013434/1357558.aspx but no luck.


I have permission (Full Control, modify, read write and others set) to following user.
[WebsiteName]_webNETWORK SERVICE[Machinename]/ftpuser

Any help on this?
 
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